Video conferences and video telephone conversations are activities which are undergoing a very fast development at the moment. Specialized journals indeed make mention of the presentation or marketing by numerous companies of videophones transmitting through ISDN (Integrated Switched Data Network) lines with a transmission rate of 48 to 64 kbits/second. This mode of transmission in a network, unfortunately, is still reserved to a fairly small circle of professional customers since the said videophones cannot be used for transmissions through the telephone network accessible to all private customers, which network has a very low transmission rate. A digitized image in fact comprises a very large quantity of information (one second of television requires, for example, a coding space of approximately 30 Mbytes, a still picture of CCIR format a coding space of approximately 1 Mbyte), which would have to be reduced by a very high compression factor.
One of the means which render it possible to reduce this quantity of information consists in a preliminary treatment of the initial pictures so as to select the most useful data to be transmitted (and/or stored). The article by W. J. Welsh, "Model-based coding of videophone images" in the journal "Electronics and Communication Engineering Journal", February 1991, pp. 29-36, proposes a preliminary treatment of this type. This document indeed describes the isolation from the pictures of the contour of the head and the shoulders, i.e. of the moving portion of the image sequence, through the use of an algorithm proposed by M. Cass, A. Witkin, and D. Terzopoulos in their article "Snakes: active contour models", in "Int. J. Comput Vision", 1988, pp. 321-331. This procedure, however, is comparatively complicated and the convergence of the algorithm, which proceeds by approximations which are to lead to a stabilization at the level of the contour of the head, is not assured.